Method and system for synchronizing colorimetric rendering of a juxtaposition of display surfaces

ABSTRACT

A method for making uniform the colorimetric rendering of a display surface including several adjacent display screens ( 10   a   , 10   b ), comprising for each pair of adjacent screens ( 10   a,    10   b ) steps of: periodically sampling ( 21 ) by a calculation device ( 5 ) image data in two corresponding screen ( 10   a,    10   b ) areas ( 13   a,    13   b ) in the pair of adjacent screens, analyzing ( 22, 23, 24 ) by the calculation device ( 5 ) image data sampled in each period to determine a difference in calorimetric rendering between the two screen areas, determining by a correction device ( 6 ) connected to the calculation device a process to be applied to the video stream to one of the two video systems controlling the two screens in the pair of adjacent screens, by applying a predetermined correction law to the difference in calorimetric rendering, and applying ( 26 ) by the correction device the process to said video system, in order to make the calorimetric rendering of the display surface uniform.

This invention relates to the display of an image or a sequence of images on several adjacent display screens, each display screen being controlled by an independent video processing system.

It is applicable particularly, but not exclusively, to the display of images such as video images on a large screen composed of several adjacent and contiguous screens forming a screen wall. In such a system, each display unit is controlled by a separate video system so as to display a corresponding part of video images. Each display unit may be composed of an overhead display system, a video projection system, or a solid screen such as a cathode ray tube screen, or a plasma or liquid crystal screen.

In particular, it is applicable to videoconference systems putting two remote sites into two-directional audiovisual communication through a high speed transmission network, each remote site being equipped with what is called a “remote presence wall” composed of several adjacent and contiguous screens. In this type of system, each display unit is controlled by a separate video system connected to a camera installed on the other remote site.

The colorimetric rendering on each screen must be uniform from one screen to the other, so that the discontinuities in the images displayed by the different screens forming the “remote presence wall” due to the fact that they are displayed by different screens, are not visible.

At the present time, the calorimetric setting of each display unit is done manually. It has been shown that this solution is not completely satisfactory, particularly due to the fact that the precision obtained by manual settings is not enough to eliminate all perceptible differences in the colorimetric rendering between two adjacent screens. Since the operator's eye is the only measurement instrument used, the evaluation of differences in calorimetric rendering remains very subjective.

Tools are available for observing and measuring display surfaces, such as colorimetric probes. However, this type of tool has never been combined with display unit adjustment systems or video processing systems.

Moreover, the operator remains dependent on the limits of the capacity to adjust the different video systems (no very large adjustment increment, adjustment inertia). All these limitations mean that manual adjustments are very approximate.

Moreover, regardless of the display technique used, the colorimetric rendering of a screen is subject to slow variations, particularly due to aging of some display unit or video system devices, which means that such adjustments have to be made regularly. Since these adjustments require a visit by an operator, it often happens that they are not carried out when they are necessary.

The purpose of this invention is to eliminate these disadvantages, but without replacing manual adjustments that are still necessary to correct large differences in colorimetric rendering between adjacent screens.

This objective is achieved by providing a method for making the colorimetric rendering of a display surface uniform, this surface including at least two adjacent display screens, controlled by video processing systems through which the corresponding video flows pass. According to the invention, this method comprises for each pair of adjacent screens in said display screens steps of:

-   -   periodically sampling by a calculation device image data in two         corresponding screen areas in the pair of adjacent screens,     -   analyzing by the calculation device image data sampled in each         period to determine a difference in colorimetric rendering         between the two screen areas,     -   determining by a correction device connected to the calculation         device a process to be applied to the video stream to one of the         two video systems controlling the two screens in the pair of         adjacent screens, by applying a predetermined correction law to         the difference in calorimetric rendering, and     -   applying the process to said video system by the correction         device, in order to make the calorimetric rendering of the         display surface uniform.

Advantageously, the sampled image data are obtained using a camera placed overlapping the two screens on each pair of adjacent screens.

According to one specific feature of the invention, the analysis of sampled image data includes steps consisting of:

-   -   considering two measurement areas located symmetrically on each         side of a separation line between the two screens in each pair         of adjacent screens, in the sampled image data,     -   determining color components in each measurement area, and     -   determining differences in color components by comparing each         average color component in one of the two measurement areas with         the corresponding average color component in the other         measurement area, the differences in color components forming         the difference in calorimetric rendering.

Preferably, the average color components in each measurement area are determined from color components on each pixel in the measurement area.

Also preferably, image data are sampled at intervals of the order of a few minutes to a few tens of minutes.

According to another specific feature of the invention, the correction law applied by the correction device to the difference in calorimetric rendering is a matrix type law.

Alternatively, the correction law applied by the correction device to the difference in calorimetric rendering may be an iterative type law with a convergence criterion.

According to yet another specific feature, the method according to the invention includes a step of comparing the difference in colorimetric rendering with a predefined threshold, and if the difference in the calorimetric rendering is greater than the threshold, no correction process is applied to the video stream of either of the two video systems controlling the two screens in the pair of adjacent screens.

Advantageously, if the difference in calorimetric rendering is greater than the threshold for several consecutive periods, an alert signal is sent indicating that a manual adjustment of the video processing systems is necessary to make the calorimetric rendering of the display surface uniform.

According to yet another specific feature, the method according to the invention includes steps of storing a history of calorimetric differences determined for each pair of display screens, and making an analysis of the history to set up a colorimetric drifts law for each display screen.

The invention also relates to a system for making the calorimetric rendering of a display surface uniform, this surface including at least two adjacent display screens, controlled by video processing systems through which the corresponding video streams pass. According to the invention, this system comprises a system for correction of the colorimetric rendering of the display screen by separation area between two adjacent screens, each correction system including:

-   -   a sensor overlapping the separation area between the two         screens, for supplying image data of images displayed on the two         screens,     -   a calculation device connected to the sensor and designed to         periodically sample image data from among the image data         supplied by the sensor, analyze the sampled image data, and         deduce from said analysis a difference in calorimetric rendering         between the two screens,     -   a correction device connected to the calculation device and         designed to determine a process to be applied to the video         stream from one of the two video systems controlling the         corresponding two screens, applying a predetermined correction         law to the difference in colorimetric rendering, and to apply         said process to the video stream in order to make the         calorimetric rendering of the display surface uniform.

Advantageously, the sensor is a video camera.

One preferred embodiment of the invention will be described below as a non-limitative example, with reference to the appended figures, wherein:

FIG. 1 shows a video image display system with two display units, equipped with the system according to the invention;

FIG. 2 shows an image taken by the sensor of the system shown in FIG. 1, for which the colorimetric rendering is analyzed in accordance with the method according to the invention;

FIG. 3 illustrates the method according to the invention in the form of a flow chart;

FIG. 4 shows a display screen with n display units, equipped with the system according to the invention.

FIG. 1 shows a video image display system with two contiguous screens 10 a, 10 b.

This system includes two video systems each comprising a video image source 1 a, 1 b connected to a display unit 3 a, 3 b, each display unit controlling one of the display screens 10 a, 10 b. The assembly consisting of a display unit 3 a, 3 b and a display screen 10 a, 10 b actually represents an overhead projection system, a video projection system, or a solid screen such as a cathode ray tube screen, a plasma screen or a liquid crystal screen.

According to the invention, a sensor 7 is placed overlapping the separation area 12 between the two screens 10 a, 10 b, that transmits colorimetric measurements to a calculation device 5 designed to determine a difference in colorimetric rendering between the two screens 10 a, 10 b. The calculation device is connected to a correction device 6 inserted in one of the video systems and designed to correct color components of the video stream passing through the system as a function of the difference in colorimetric rendering determined by the calculation device 5. In the example illustrated in FIG. 1, the correction device 6 is placed between the video source 1 b and the display unit 3 b so as to modify color component parameters of the video stream from the source 1 b, before applying them to the display unit 3 b.

For example, the sensor 7 is composed of a video camera that transmits a video stream, or a periodic digital image, to the calculation device 5.

In accordance with the method according to the invention illustrated in FIG. 3, the calculation device 5 samples 21 one or several images 11 of the video stream at regular intervals. Such an image is illustrated in FIG. 2. Each of these images represents part of the separation zone between the two contiguous screens 10 a, 10 b, and part of the images displayed by these two screens.

The calculation device analyses sampled images considering two measurement areas 13 a, 13 b distributed symmetrically on each side of the separation area 12 between the two screens. For example, the two measurement areas 13 a, 13 b may be squares with p×p pixels, and 3×3 pixels in the example shown in FIG. 2.

Since the colorimetric drift of display units is generally a relatively slowly changing phenomenon, there is no need to carry out the processing done by the calculation device 5 continuously. A corrective processing done at intervals of between a few minutes and a few tens of minutes is sufficient.

The analysis performed by the calculation device 5 consists of determining 22 the corresponding values of the R (red), G (green) and B (blue) components for each pixel in each area 13 a, 13 b, according to the units used in the computer domain. These values may for example be determined in the form of binary values on 8 bits.

The values of the color components thus determined are then weighted and averaged 23 on all pixels in each area 13 a, 13 b to obtain global color components Ca_(R), Ca_(G), Ca_(B) and Cb_(R), Cb_(G), Cb_(B) for each of these areas. The calculation device may also calculate an average of these values on several sampled images.

The global components Ca_(R), Ca_(G), Ca_(B) and Cb_(R), Cb_(G), Cb_(B) obtained for each zone 13 a and 13 b are then compared 24 in pairs to determine a calorimetric difference between the two screens, this difference consisting of a difference value for each color component. For example, this difference may be in the form (ΔC_(R), ΔC_(G), ΔC_(B))=(Cb_(R)−Ca_(R), Ca_(G)−Cb_(G), Ca_(B)−Cb_(B)).

The value of this calorimetric difference is transmitted to the correction device 6 that uses a correction law f(ΔC_(R), ΔC_(G), ΔC_(B)) to deduce 26 the correction processing to be applied to the video stream to be corrected, in other words the video stream output from the source 1 b in the example in FIG. 1, and then applies this processing.

The correction law f(ΔC_(R), ΔC_(G), ΔC_(B)) may be of the matrix or iterative type using a convergence criterion.

A matrix type correction law consists of applying the calculated difference in colorimetric rendering (ΔC_(R), ΔC_(G), ΔC_(B)) or a function of this difference to each pixel in the images of the video stream to be corrected.

An iterative type correction law consists of applying the difference in colorimetric rendering (ΔC_(R), ΔC_(V), ΔC_(B)) to each pixel of images in the video stream to be corrected, and consecutively remeasuring the new difference (ΔC_(R)′, ΔC_(V)′, ΔC_(B)′), checking that it is less than the previously measured difference, and applying this new difference to all pixels. The convergence criterion is therefore: ΔC_(R)′<ΔC_(R), ΔC_(V)′<ΔC_(V), ΔC_(B)′<ΔC_(B). This processing is repeated until a difference in colorimetric rendering less than a predetermined threshold is obtained, this threshold being advantageously the perception threshold of the human eye. It the convergence criterion is not satisfied, then the iterative method is not appropriate and the matrix correction method is applied.

Obviously, these processings may be done in other systems of units, such as the YUV video units system or the system of XYZ units used for the human eye. Advantageously, it would be possible to use known transformation matrices to convert values of all components in RGB into other systems of units that are better adapted to the video system to be corrected.

Preferably, the values of differences measured on each color component is compared 25 with a corresponding predetermined threshold value, so as to be able to determine if these differences are acceptable by the correction device 6, in other words if they are not too large to be corrected by this correction device.

If these differences are incompatible with correction possibilities available in the correction device 6, the correction device will not apply any correction to the video stream output from the source 1 b. If these differences remain unacceptable during several consecutive measurement periods, the system sends 29 an alert signal to indicate that manual action will be necessary on the adjustment of the display units or the video systems. The system may increment 27 a counter C for this purpose and when the value of this counter exceeds 28 a certain threshold, the system sends an alert signal. Obviously, the value of the counter C will be reset to zero if a correction is made.

This arrangement means that a difference in color between images displayed on the two screens 10 a, 10 b resulting from the passage of an object in front of the camera(s) will not be taken into account, and when the edge of an object is displayed between the two measurement areas 13 a, 13 b.

The system described with reference to FIG. 1 may be generalized to n display screens, as shown in FIG. 4. In this figure, the display system comprises n display screens 10 (where n is an integer number more than 2), each screen being controlled by a corresponding display unit 3. The correction system according to the invention comprises a colorimetric measurement and correction system using the separation line 12 between two adjacent screens. Each rank i measurement and correction system (where i is an integer number between 1 and n−1) includes a sensor 7 positioned facing a separation line 12 between two adjacent screens 10 with ranks i and i+1, this sensor being coupled to a calculation device 5 calculating the rank i colorimetric drift, which is connected to a rank i correction device 6, inserted in the control video system of the rank i+1 display unit 3. Therefore this system includes n−1 calorimetric measurement and correction systems, the rank 2 to n display units 3 being adjusted one by one starting from the rank 1 screen 10.

It would be possible to store a history of differences calculated between the two measurement areas, and analyzing this history using a statistical tool 7 (FIG. 1) so as to create colorimetric drift laws as a function of the display unit types and models. These calorimetric drift laws may be used to anticipate and therefore plan operator actions to make manual adjustments. 

1. A method for making the calorimetric rendering of a display surface uniform, said display surface including at least two adjacent display screens (10 a, 10 b) controlled by video processing systems through which the corresponding video flows pass, characterized in that it includes for each pair of adjacent screens (10 a, 10 b) in display screens the steps of: periodically sampling (21) by a calculation device (5) image data in two corresponding screen (10 a, 10 b) areas (13 a, 13 b) in the pair of adjacent screens, analyzing (22, 23, 24) by the calculation device (5) image data sampled in each period to determine a difference in colorimetric rendering between the two screen areas, determining by a correction device (6) connected to the calculation device a process to be applied to the video stream to one of the two video systems controlling the two screens in the pair of adjacent screens, by applying a predetermined correction law to the difference in colorimetric rendering, and applying (26) by the correction device the process to said video system, in order to make the colorimetric rendering of the display surface uniform.
 2. The method according to claim 1, characterized in that the sampled image data are obtained using a camera (7) placed overlapping the two screens (10 a, 10 b) on each pair of adjacent screens.
 3. The method according to claim 1, characterized in that the analysis of sampled image data comprises steps of: considering in the sampled image data two measurement areas (13 a, 13 b) located symmetrically on each side of a separation line (12) between the two screens (10 a, 10 b) in each pair of adjacent screens, determining (22, 23) average color components in each measurement area, and determining (24) differences in color components by comparing each average color component in one of the two measurement areas with the corresponding average color component in the other measurement area, the differences in color components forming the difference in calorimetric rendering.
 4. The method according to claim 3, characterized in that the average color components in each measurement area (13 a, 13 b) are determined (23) from color components on each pixel in the measurement area.
 5. The method according to claim 1, characterized in that image data are sampled (21) at intervals of the order of a few minutes to a few tens of minutes.
 6. The method according to claim 1, characterized in that the correction law applied by the correction device (6) to the difference in calorimetric rendering is a matrix type law.
 7. The method according to claim 1, characterized in that the correction law applied by the correction device (6) to the difference in colorimetric rendering is an iterative type law with a convergence criterion.
 8. The method according to claim 1, characterized in that it comprises a step (25) of comparing the difference in colorimetric rendering with a predefined threshold, and if the difference in the colorimetric rendering is greater than the threshold, no correction processing is applied to the video stream of either of the two video systems controlling the two screens (10 a, 10 b) in the pair of adjacent screens.
 9. The method according to claim 8, characterized in that if the difference in colorimetric rendering is greater than the threshold for several consecutive periods, an alert signal is sent (29) indicating that a manual adjustment of the video processing systems is necessary to make the colorimetric rendering of the display surface uniform.
 10. The method according to claim 1, characterized in that it comprises steps of storing a history of colorimetric differences determined for each pair of display screens, and making an analysis of the history to set up a colorimetric drifts law for each display screen.
 11. A system for making the calorimetric rendering of a display surface uniform, this surface including at least two adjacent display screens (10 a, 10 b), controlled by video processing systems through which the corresponding video streams pass, characterized in that it comprises a system for correction of the calorimetric rendering of the display screen by separation area (12) between two adjacent screens (10 a, 10 b), each correction system including: a sensor (7) overlapping the separation area between the two adjacent screens, for supplying image data of images displayed on the two screens, a calculation device (5) connected to the sensor and designed to periodically sample image data from among the image data supplied by the sensor, to analyze sampled image data and to deduce a difference in colorimetric rendering between the two screens from this analysis, a correction device (6) connected to the calculation device and designed to determine a process to be applied to the video stream from one of the two video systems controlling the corresponding two screens (10 a, 10 b), applying a predetermined correction law to the difference in colorimetric rendering, and to apply said process to the video stream in order to make the colorimetric rendering of the display surface uniform.
 12. The system according to claim 11, characterized in that the sensor (7) is a video camera. 